Piezoelectric ceramic compositions consisting essentially of lead titanate zirconate or lead titanate are widely used in piezoelectric ceramic devices such as piezoelectric ceramic filters, etc.
Known as piezoelectric materials are compounds containing Ba, Sr, Na, Nb, O and the like. For example, Ba.sub.2 NaNb.sub.5 O.sub.15, Sr.sub.2 NaNb.sub.5 O.sub.15 and the like are known as materials having piezoelectric characteristics and electro-optical effects, and single crystals of those compounds have been being widely studied. However, few studies have been made on the use of those compounds. In 1989, J. R. Oliver et al. (J. Am. Ceram. Soc., 72 2! 202-11 (1989)) reported the ferroelectricity of ceramics of (Ba.sub.1-x Sr.sub.x).sub.2 NaNb.sub.5 O.sub.15 (0.ltoreq.x.ltoreq.1) , but they did not refer in detail to the piezoelectric characteristics of those ceramics.
Conventional piezoelectric ceramic compositions consisting essentially of lead titanate zirconate or lead titanate, which have heretofore been used widely, contain lead, and are therefore not so favorable from the viewpoint of environmental protection. To produce the conventional compositions, generally used are lead oxides. The lead oxides used easily vaporize, resulting in that it is relatively difficult to control the quality of the products produced.
On the other hand, ceramics of (Ba.sub.1-x Sr.sub.x).sub.2 NaNb.sub.5 O.sub.15 (0.ltoreq.x.ltoreq.1) do not contain lead and are therefore free from the problems mentioned above. However, those ceramics are inferior to the piezoelectric ceramic compositions consisting essentially of lead titanate zirconate or lead titanate in point of their piezoelectric characteristics as indicated by, for example, the electromechanical coupling factor and the piezoelectric constant, and therefore they have not been practiced industrially as materials for piezoelectric ceramic devices such as piezoelectric ceramic filters, piezoelectric ceramic oscillators, etc.
Given that situation, the object of the present invention is to improve the piezoelectric characteristics of those (Ba.sub.1-x Sr.sub.x).sub.2 NaNb.sub.5 O.sub.15 (0.ltoreq.x.ltoreq.1) ceramics and to provide a novel piezoelectric ceramic composition which does not contain lead and which can be used industrially as a material for piezoelectric ceramic devices such as piezoelectric ceramic filters, piezoelectric ceramic oscillators, etc.